The inability to find compatible blood for transfusion is not an uncommon occurrence. A procedure has been developed whereby a small amount of 51Cr labeled donor red cells which are incompatible by in-vitro techniques are transfused into the potential recipient and the in-vivo survival of the transfused cells determined. Survival characteristics of these cells allow decisions to be made with greater certainty regarding the safety of standard transfusions. In the few patients studied thus far, a rare case of "saline auto-agglutinating phenomenon" has been shown to be of no significance clinically, while the suspected incompatibility of Group O red cells with a rare "Bombay O" recipient has been documented. In other instances "incompatible" blood for important surgery has been able to be released, whereas previously it would have been difficult to predict the relative hazard of transfusing such blood. It is anticipated that further patients will be included in this study when in vivo crossmatch incompatibility is encountered.